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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.overview.core.allocation"></a><a class="link" href="allocation.html" title="Custom Memory Allocation">Custom Memory
        Allocation</a>
</h4></div></div></div>
<p>
          Many asynchronous operations need to allocate an object to store state
          associated with the operation. For example, a Win32 implementation needs
          <code class="computeroutput"><span class="identifier">OVERLAPPED</span></code>-derived objects
          to pass to Win32 API functions.
        </p>
<p>
          Furthermore, programs typically contain easily identifiable chains of asynchronous
          operations. A half duplex protocol implementation (e.g. an HTTP server)
          would have a single chain of operations per client (receives followed by
          sends). A full duplex protocol implementation would have two chains executing
          in parallel. Programs should be able to leverage this knowledge to reuse
          memory for all asynchronous operations in a chain.
        </p>
<p>
          Given a copy of a user-defined <code class="computeroutput"><span class="identifier">Handler</span></code>
          object <code class="computeroutput"><span class="identifier">h</span></code>, if the implementation
          needs to allocate memory associated with that handler it will execute the
          code:
        </p>
<pre class="programlisting"><span class="keyword">void</span><span class="special">*</span> <span class="identifier">pointer</span> <span class="special">=</span> <span class="identifier">asio_handler_allocate</span><span class="special">(</span><span class="identifier">size</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">h</span><span class="special">);</span>
</pre>
<p>
          Similarly, to deallocate the memory it will execute:
        </p>
<pre class="programlisting"><span class="identifier">asio_handler_deallocate</span><span class="special">(</span><span class="identifier">pointer</span><span class="special">,</span> <span class="identifier">size</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">h</span><span class="special">);</span>
</pre>
<p>
          These functions are located using argument-dependent lookup. The implementation
          provides default implementations of the above functions in the <code class="computeroutput"><span class="identifier">asio</span></code> namespace:
        </p>
<pre class="programlisting"><span class="keyword">void</span><span class="special">*</span> <span class="identifier">asio_handler_allocate</span><span class="special">(</span><span class="identifier">size_t</span><span class="special">,</span> <span class="special">...);</span>
<span class="keyword">void</span> <span class="identifier">asio_handler_deallocate</span><span class="special">(</span><span class="keyword">void</span><span class="special">*,</span> <span class="identifier">size_t</span><span class="special">,</span> <span class="special">...);</span>
</pre>
<p>
          which are implemented in terms of <code class="computeroutput"><span class="special">::</span><span class="keyword">operator</span> <span class="keyword">new</span><span class="special">()</span></code> and <code class="computeroutput"><span class="special">::</span><span class="keyword">operator</span> <span class="keyword">delete</span><span class="special">()</span></code> respectively.
        </p>
<p>
          The implementation guarantees that the deallocation will occur before the
          associated handler is invoked, which means the memory is ready to be reused
          for any new asynchronous operations started by the handler.
        </p>
<p>
          The custom memory allocation functions may be called from any user-created
          thread that is calling a library function. The implementation guarantees
          that, for the asynchronous operations included the library, the implementation
          will not make concurrent calls to the memory allocation functions for that
          handler. The implementation will insert appropriate memory barriers to
          ensure correct memory visibility should allocation functions need to be
          called from different threads.
        </p>
<h6>
<a name="boost_asio.overview.core.allocation.h0"></a>
          <span><a name="boost_asio.overview.core.allocation.see_also"></a></span><a class="link" href="allocation.html#boost_asio.overview.core.allocation.see_also">See
          Also</a>
        </h6>
<p>
          <a class="link" href="../../reference/asio_handler_allocate.html" title="asio_handler_allocate">asio_handler_allocate</a>,
          <a class="link" href="../../reference/asio_handler_deallocate.html" title="asio_handler_deallocate">asio_handler_deallocate</a>,
          <a class="link" href="../../examples.html#boost_asio.examples.allocation">custom memory allocation
          example</a>.
        </p>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2012 Christopher M. Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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